The present invention relates to a color cathode ray tube and more particularly to a color cathode ray tube in which beam landing errors caused by thermal expansion of the shadow mask structure are reduced without reducing the strength of the shadow mask.
A cathode ray tube used for displaying a color image has a glass envelope which includes a panel portion on which a display screen is disposed, a neck portion accommodating an electron gun, and a funnel portion connecting the panel portion and the neck portion. At the funnel portion there is installed a deflection device that scans an electron beam emitted from the electron gun over a phosphor screen formed on the inner surface of the panel portion.
The electron gun accommodated in the neck portion has a variety of electrodes, such as a cathode electrode, a control electrode, a focus electrode and an accelerating electrode. The electron beam emitted from the cathode electrode is modulated by a signal applied to the control electrode and is passed through the focus electrode and the accelerating electrode to be formed into a desired cross-sectional shape and is given a desired energy before impinging on the phosphor screen. The electron beam, on its path from the electron gun to the phosphor screen, is deflected horizontally and vertically by the deflection device installed on the funnel portion to form an image on the phosphor screen (Japanese Patent Laid-Open No. 215640/1984).
When the electron beam emitted from the electron gun is subjected to color selection by a shadow mask disposed inside the panel portion so that its portion corresponds to the color phosphors that are selected, a part of the beam other than that which strikes the phosphor screen impinges on the shadow mask. When the electron beam strikes the shadow mask, a part of the energy of the electron beam is converted into thermal energy, which in turn raises the temperature of the shadow mask causing thermal expansion thereof.
The shadow mask is formed in the shape of a dome curving toward the panel side and is surrounded by a frame along its periphery. Hence, when the shadow mask undergoes thermal expansion, it expands towards the phosphor screen side, and a so-called doming phenomenon occurs.
Common measures against the doming phenomenon include the use of an invar material with a low thermal expansion coefficient for the material of the shadow mask, the use of a bimetal for the spring supports for the shadow mask, and the shortening of the skirt portion of the shadow mask.
When invar material having a low thermal expansion coefficient is used for the material of the shadow mask, the doming of the shadow mask, even with a nearly flat shape, can be prevented because the invar itself has a low thermal expansion characteristic.
The invar material used for the shadow mask with a low thermal expansion coefficient exhibits a strong spring phenomenon and thus, when it is press-formed, spring-back occurs at the skirt portion of the shadow mask, making it difficult to press the skirt portion into a shape corresponding to the design values. Further, the invar material has a high cost, thereby raising overall the cost of the color cathode ray tube.
When a bimetal element made by joining together materials with different thermal coefficients is used for the spring support members which hold the shadow mask structure, the warping of the bimetal material as a result of temperature variation can be utilized to move the shadow mask structure to match the position of the electron beam passing openings with the electron beam passing positions to balance the purity drift and the ambient temperature duff.
The use of a bimetal material for the mask spring support members, however, has a drawback. During the operation of the color cathode ray tube, when the ambient temperature of the shadow mask structure increases, the entire shadow mask structure moves more than required to achieve correct compensation, which in turn causes beam landing errors.
An example of a construction in which the shadow mask has a short skirt portion between the welded fixed portions is found in Japanese Patent Laid-Open No. 22048/1992.
In this shadow mask construction, although the doming characteristic can be improved by making the skirt portion between the welded fixed portions shorter than the welded fixed portions, the narrow skirt width between the welded fixed portions of the shadow mask results in a reduced strength, making the handling of the shadow mask structure during the manufacturing process difficult.
Particularly for shadow masks used in large cathode ray tubes, a sufficiently large strength is required to prevent the shadow mask from being deformed.
Increasing the skirt width for a greater strength reduces the doming reduction effect due to the thermal expansion of the skirt portion. The deformation of the shadow mask structure and the doming phenomenon entails a beam landing shift, deteriorating the color purity.
In addition, a color display tube of the type used in a color monitor is required to have an increased number of horizontal scanning lines compared with the color cathode ray tubes used for color television, which in turn increases the amount of heat produced in the shadow mask and the deflection yoke. The color impurity caused by the doming phenomenon due to heating becomes more serious in this case and produces a major problem in a high definition cathode ray tube that has a shadow mask hole pitch of 0.31 mm or less, which determines the dot pitch of the phosphor screen, or has substantially 1,000 or more horizontal scanning lines.
The present invention is intended to solve the above problem and its objective is to provide a color cathode ray tube that has a shadow mask with a strength sufficient to prevent deformation and with a small strain and that has reduced beam landing errors.